Personally, I strive to keep all my goods inside the pack, including sleeping pad. This serves to protect gear from weather, muck and abrasion, and helps insure that it won't be lost. There are times when I succumb to lashing doo-dads on the outside, but I much prefer to avoid this.

A compression sack may hasten a sleeping bag's inevitable loss of loft. This may be especially true with synthetic insulation. I formerly used one regularly with a minus-20F -rated synthetic bag, and found it invaluable, despite the damage it eventually seemed to cause.

I did not find a compression sack necessary for a 20F-rated synthetic sleeping bag, which I used for a couple of seasons, and which nonetheless, pretty rapidly lost a significant amount of loft. It fit into a 4,000 cubic-inch pack, leaving room, barely, for a week's worth of food and gear. Still, a compression sack is worth considering.